Phosphoric acid manufacture



Sept. 11,r 1945. M. sHoELD PHOSPHORIC AC ID MANUFACTURE Filed Jan. 15, 1941 5 Sheets-Shee'tI l mu Hwmwj Sept. l1, 1945. M, sHoELD PHOSPHORIC ACID MANUFACTURE Filed Jan. 15, 1941 5 Sheets-Sheet 2 Sept. ll, 1945. M, sHoELD PHOSPHORIC ACID MANUFACTURE v5 sheets-sheet s Filed Jan. '15. 1941 gmc/whom:

Sept. l1, 1945. M, SHOELD PHosPHoRIc ACID MANUFACTURE Filed Jan. 15, 1941 5 Sheets-Sheet 4 5 Sheets-Sheet 5 Sept. 11, 1945. M. sHoELD PHOSPHORIC ACID MANUFACTURE Filed Jan. 15, 1941 wmf W/MM.

. *MDAX Patented Sept. 1l, 1945 rnosrnomc Acro MANUFACTURE Mark Shoeld, Towson, Md., assignor. to The Davison Chemical Corporation, Baltimore, Md., a corporation of Maryland Application January 15, 1941, Serial No. 374,591

18 Claims.'

This invention relates to the manufacture of phosphoric acid and more particularly to a procl ess of extracting or leaching phosphoric acid, and y apparatus therefor.

Numerous methods have heretofore been suggested for the preparation of orthophosphoric acid including for instance the production of phosphorus in an electric or blast furnace and burning it to pentoxide and the subsequent absorption of the phosphorus pentoxide in water; and the so-called wet processes in which phosphate rock or tri-calcium phosphate is treated with acids such as sulphuric, hydrochloric and nitric acids. Ordinarily in wet processes that have reached the commercial stage, phosphate rock is mixed with sulphuric acid solution to Iform a slurry and the slurry is i'lltered or permitted to settle to separate solid unleached material from phosphoric acid solution. A weak phosphoric a'cid solution of from 20 to 30% P205 is obtained, which must be concentrated by evaporation of the excess water.

In the present process superphosphate, previously prepared from phosphate rock, is treated in stationary beds with sulphuric acid solution that is diiused throughoutl such beds, wherein it is observed that positive mechanical mixing or stirring and heat are unnecessary in order to accomplish the desired objects. Furthermore, the 'final product', is a phosphoric acid solution of high concentration of from 40% to 50% P205 obtained direct and Without any evaporation, and substantially free from calcium sulphate. Also, although the phosphoric acid solution is not filtered, the product is substantially clear and of relatively high purity. Other advantages will be made apparent in the description of the process given hereinbelow.

Essentially, the chemical reaction involved is between sulphuric acid and the monocalcium phosphate of the superphosphate wherein phosphoric acid is liberated and calcium sulphate forms. The sulphuric acid penetrates the particles of superphosphate and the ,phosphoric acid is recovered by leaching Vwith a solvent such as water. A

A superphosphate product that is particularly usefulin the preparation of phosphoric-acid in the present processbecause of the -high porosity and the relatively high compression strength and other characteristics oi'granules of superphosphate, is the superphosphate product described and claimed as distinct from den superphosphate in Reissue Patent No. '19,825, dated January 14,

superphosphate product may be manufactured by the process described and claimed in Patent No. 2,136,793, dated November l5, 1938, issued to W. H. Gabeler, A. C. Wilson, T. O. Tongue, and M. Shoeld. These particular products are pre- 5 pared by iirst preliminarily conditioning superphosphate by tumblingthe same in the presence of an aqueous medium in amount suilicient to cause nodulization and in a substantially non/- drying atmosphere while moving through a rotary container at atmospheric pressure until the mass is agglomerated to rm nodules. Directly .thereafter, the material thus conditioned is heated in adrying atmosphere while tumbling the same at atmospheric pressures in a stream moving through a rotary container' until converted to discrete, nodular, encrusted and indurated globoid particles. If desired these particles may be screened to obtain a suitable mesh (for instance material passing through a 3 mesh screen but retained on a screen having .0137 inch openings) or the particles may be ground to obtain this mesh. A satisfactory mesh is from about 4 to 20.

The volumetric porosity of a granule obtained by the above patented process is about thirty per cent of the total volume of the granule.

A typical analysis of the superphosphate grany `ules obtained by the above patented process is as follows:

Percent Moisture 1.5 to 2.o

TOtal P205 20.5 t0 21 Insoluble P205 0.4 to 0.8 Available P205 20.2 t0 20.6

As is true of the above patented superphosphate, in the present process a superphosphate must be employed which does not pack or'cake when in deep piles.

By way of illustration, reference is made to 40 the accompanying drawings in which apparatus is shown by which the process is eillcientlyconducted.

Figure l is an elevational View of a plant showing four of the eight leaching tanks in the row in the upper part of Fig. 2, and diagrammatically depicting the piping and material conveying. systems employed;

Fig. 2 is a plan view of therplant showing eight leaching tanks andal diagrammatic representation of the piping arrangement therefor;

Fig. 3 is an elevational view of the plant taken NonIineL-SofFIgJ;

Fig. 4 is a vertical section of a leaching tank, and includes a diagrammatic representation of 1936, issued to B. Ober landE. H. Wight, which the piping associated with the tank;

- Fig. 5 is a sectional view of apparatus shown in Fig. d taken on line 55;

Fig. 6 is a section of a fragment of the apparatus; and

Figs. '7, 8 and 9 are flow diagrams illustrating dierent stages in the process.

Similar parts of the apparatus are designated by the same reference characters.

The apparatus shown comprises a plurality Yof tanks details of which are shown in Figs. 'i

and 5, and means by which the various steps in the leaching process may be conducted. Althrough eight leaching tanks numbered consecutively l to 8 are shown, the number employed may vary. Six tanks or as many as twelve or more may be conveniently employed in the process. Eight tanks have been found highly practical for commercial purposes. Each tank is of sulcient capacity and shaped to hold a relatively deep bed of phosphate material. Tanks in which as much as 50 to 100 tons or more of superphosphate may be treated at one time, are found desirable.

A leaching tank is constructed of an outer casing it of steel lined with acid resistant brick il and interlined with a mastic material i2 including a bitumen or rubber. A false bottom it is provided which is raised slightlyfrom the botn tom of the tank by supporting means lli. The false bottom is preferably constructed of wood and is provided with perforations i5 through which liquid may pass into the space" it. The false bottom i3 may be divided into sections as shown. in Fig. 5. A coco matting lll, suitably weighted down by lead weights I8, is superposed on the false bottom I3. This matting, although not entirely necessary, assists in distributing the liquid passing through the bottom i3, and in protecting the surface of the latter.

A leaching tank when charged for operation is filled with solid and liquid almost to the top deserve tank and as the liquid rises the valve se is op erated to increase the supply of air. At a predetermined lower level of the liquid the valve is closed to permit accumulation of liquid in the tank. If desired, well known means may be provided for properly directing any accidental overow of liquid from a leaching tank.

Referring to Figs. l, 2 and 3, leaching ta 7s i-d may be positioned relative to each other as desired. Means are provided above the tops of the tanks ifor depositing superphosphate selectively in any of these tanks. For this purpose hoppers 35 and 3S are conveniently placed so that material deposited therein may be passed down a removable chute 31 into a leaching tank. The hoppers 3d and `36 are fed by a reversible belt to which may direct the superphosphate to either one of these hoppers. The superphosphate is originally placed in a V-shaped hopr as represented by the dotted lines i9 and 20 (Fig.

4) respectively. A. shallow box 2l is supported inside the tank near the top thereof so that the upper edge of its side wall is positioned slightly above the predetermined level 2@ of the liquid in recting a stream of water under high pressure through the manhole and into the body 0f the spent solid material, whereby a slurry is formed which readily flows and is conducted away, as further described below.

A pipe 25 having an open end 26 positioned in the space it, is connected to an air-lift 2l whereby liquid `from the bottom of a leaching tank may be raised to an upperlevel for distribution selectively to several different parts of the plant as hereinafter set forth.

For the purpose of controlling the flow of liquid in the air-lift, a float 28 is provided in each leaching tank for operating a valve 29 in the air supply pipe B for the air-lift 2l. The float 2t is positioned in the liquid at the top of the leaching 3@ positioned above a belt lil which may be a weighing belt. The superphosphate feed rate is adjusted by raising or lowering a leveling gate such as a well known piano box for controlling rate of ow, or a combined head varying means and flow control is connected toA each acid tank. Each acid tank is provided with a iioat l5 and an electric signalling means is for indicating when the acid fed to the tank reaches a desired level.

'A water tank 4l is likewise positioned above the tops of the leaching tanksf for supplying water thereto. A standard control means lil similar to control means M is likewise provided to regulate the head and also the rate of flow of the water passedV into the leaching tanks.

Between the two rows of leaching tanks i-i and li-B, as shown. are a plurality of manifolds 5i, 52, 53, 54, and 55 (diagrammatically illustrated). A pipe 56 connects the control means dii to the manifold 5l for passing water thereto. A plurality of off-take pipes El! (each provided with a valve 59), one for each leaching tank i-B, are connected to the manifold 5| and have their open ends in the mixing boxes 2 l.

A pipe 69 connects the control means lil to the manifold 52 for passing acid thereto. A plurality of o`take pipes 60, each provided with a valve Si, are connected to the acid manifold 52. Means are provided whereby a given quantity of acid may be distributed through two or more 'off-takes 60 into two or more leaching tanks in equal portions. To accomplish this, each ofi-take pipe 50 extends upwardly through the bottom of a small box 63 having a removable cover 64, and shown in detail in Fig. 6. The open end.- of the pipe is readilyaccessible for the placement thereon of means for varying the size of the oritice. Such means comprise rings which iit onto the end of the pipe B0 and sleeve into each 'other and are readily stacked on each other in varying numbers. The number of rings and the selected size of the orice E6 in the rings depend upon the number of tanks among which the acid is t0 be distributed. The greater the number of tanks the greater' the number of rings used on each pipe 60.

With a given uniform head of acid, and with the open end of each pipe 60 at exactly the same level, andwith the same number of rings mounted on each pipe, a measured quantity of `the acid is .the leaching tanks. An operator learns by experience how many rings are needed for dividing the acid into separate streams to be fed si multaneously to any given number of leaching tanks. Leading from the bottom of each box Bs is a pipe @l the open end oi which is positioned to direct acid into the mixing box 2i in each leachingtank.

The air lift 2l, supplied by air through pipe td which in turn is connected to the manifold dd, extends upwardly to a stuiing box dit in which is mounted one end of a swinging elbow ld. The other end of the elbow is connected to a small funnel-like, feeding chamber llt pro-y vided with a vent 'l2 and. an outlet pipe l@ in its tapered bottom. The outlet pipe 'it and an endpiece 'it of the pipe di are directed downwardly toward each other when the'feeding chamber ifi is positioned above the mixing bor, 2B, so that the acid stream from the end-piece id and, the liquid stream from the outlet it impinge upon each other upon entering the mixing "box, whereby the liquid and. acid are premixed before entering a leaching tank, as illustrated in detail in Figd.

Two upright' pipes 15- and 1d, each connected at its upper free end to a funnel-shaped receiver 'I8 and 19 respectively, are connected at their lower ends respectively to the manifold B8 and to the lower end of the air-lift pipe 21 to which the leaching tank outlet pipe 25 is connected. The open tops of the receivers 18 and 18 are so positioned that the chamber LlIl may be swung in an arc so as to place it above either receiver. Thus liquid pumped by means of an air-lift 21 may be selectively passed into 'a mixing box 2| from one leaching tank to the next in a series, or into a receiver 18 and thence into the manifold I8, or into a receiver 18 for the purpose of introducing liquid from one leaching tank into the bottom of the next leaching tank in a series or even to bypass a leaching tank in the series.

The manifold 88 serves to conduct extracted phosphoric acid solution from the various receivers 1l into collecting tanks 82 or 88 or both. 'I'he latter tanks are interconnected and each has a capacity of about one-hal! that of a leaching tank for the purpose set forth below. Each tank 82 and 88 is provided with a lloat 8l and signal means 85 operated by the float to indicate when these tanks are full. Valves 88 and 81 are provided for cutting of! the supply of phosphoric acid to the tanks 82 and 83 respectively.

Each of the tanks 82 and 88 is connected at its bottom to the manifold 55, and to a. pipe 88 for passing phosphoric acid to storage. Pipes 8.8 each having a valve 80 connect the manifold 8B to each pipe 18 so that solution from the tanks 82 and 88 can be returned to any of the leaching tanks.

Each leaching tank is so positioned that spent material therefrom is readily conducted from the manholes 22 by means of chutes 8| into a cement trough 82. A hose (not shown) attachable to a high pressure water main 88 may be used for introduction into a manhole for the purpose of hydraulically removing solid spent material from a leaching tank. .For this purpose a stream of water is also kept moving in the trough 82 at the rateo about 250 gallons per minute.` The spent material in the form of a slurry is pumped away by either pump I8 or 88.

tus that contact acid are made of acid-resistant material.

In connection with the description of the process of the present invention reference may be made to the legend on page 1 of the drawings indicating by means of symbols the paths taken by the different materials in a plant under operation.

.Although a considerable number of variations in the operation of the disclosed apparatus is possible, the following procedure is given by way of an example oi' a highly practical manner in which the various steps in the process may be conducted:

To commence operations in a plant having eight leaching tanks (as shown in the drawings) in each of which about 40 tons of superphosphate are treated at one time, sulphuric acid (98% concentration) is passed from one of the measuring tanks i2 or 43 into leaching tank i along with water from the tank dl in such proportion that about a 25 B. solution is formed. When the tank i is about one-half full or enough acid solution has been introduced so that al charge of superphosphate is completely covered thereby, the superphosphate is run from the hopper 35 into tank i previously partly filled -with the acid solution.

broken and a thorough initial mixture of acid and superphosphate is formed. The proportion of the 98% acid to the superphosphate used in this initial mixture is about 18 pounds of the 98% acid to 100 pounds of superphosphate.

The flow of Water into tank I is continued and the passage of the 98% acid thereto is discontinued, while solution from the bottom of tank l along with sulphuric acid (98%) is passed into tank 2. Enough of this mixture is passed into tank 2 so that when the charge of superphosphate is deposited therein the charge is completely covered by said mixture. Also, the solution from tank I and the sulphuric acid (98%) fed to tank 2 are so apportioned that there will be 18 pounds of the acid to 100 pounds of the superphosphate (having the analysis indicated above) in the charge for tank 2.

In charging tank 8, the sulphuric acid for this charge may be apportioned between tanks 2 and 3 and the solution from tank 2 run into tank 3 until about one-half full. Superphosphate is then charged into tank 8. This is repeated for tanks 45, 8 and 1, the water from the tank 41 being at all times continuously fed into tank l at a predetermined while acid from tank 42 or 43 is continuously fed proportionately at a predetermined rate into one or more intermediate tanks, or intoy all tanks except tank l and 8. After tank 1 is charged, solution alone from tank 'l is passed from the bottom thereof into tank 8 until the latter is one-half full. Superphosphate is then deposited in tank 8 so that the tank is filled to the top withl jdescription of the apparatus, when the solution from tank 8 is being passed to a collecting tank 82 The various conduits and parts of the apparaor 88, the feeding chamber Il for tank l is swung into position above the receiver 18 shown in Figs. l and 2 to .the left of tank l, thus conducting phosphoric acid solution into the manifold 53.

The rate of ow of water during operation of the plant in an eight hour cycle with eight leaching tanks each having a capacity for treating 40 tons of superphosphate at one time is maintained in the weak end of the series at about to 12 gallons per minute. This would be correspondingly changed for plants of different capacities and operating under a 10 or`l2 hour or other cycle. The acid rate of flow is also maintained at a constant, uniform iiow such that about 18 pounds of sulphuric acid (98%) are used for every 100 pounds of superphosphate introduced into each cycle, the superphosphate containing about 20.5 to 21 per cent total P205.

During the passage of solution from one leaching tank to the next in the series in an 8 hour cycle, the fiow is maintained at a rate equivalent to the passage of about'sixty pounds of the solution per 100 pounds of superphosphate over about an eight hour period. This flow is controlled by properly adjusting the air pressure in the manifold 54 and the valves 29 in the air pipes 30 controlled by the floats 28 in each leaching tank. A pressure regulator 98 and a pressure indicator 99 may also be provided in the manifold 54 for conltrol purposes.

The diagrams in Figs. 7, 8 and 9 illustrate three successive stages in a cycle after operation is established. In Fig. 7', tank i has just been temporarily taken out of the cycle to remove spent material. Water alone is 110W being passed into tank 2. As will be seen in later stages, water is introduced into the tank that succeeds the tankr that is being emptied of spent material and filled with fresh superphosphate.

In the cycle illustrated in Fig. 7, while flow is maintained from the bottom of one tank into the top of the next', sulphuric acid (98%) is being the fresh superphosph'ate charge is deposited in the leaching tank. In this way any unreacted sulphuric acid present in the phosphoric acid solution is certain to be used up in contact with the fresh superphosphate.

In the stage shown in Fig. 7, the solution from the bottom of tank 'l is passed into tank 8 on top of the charge therein without sulphuric `acid from the acid tanks 42 and 43. `Sulphuric acid carried from tank I with the leach'ed phosphoric acid in its higher concentration is also relatively quickly used up so that the solution initially at least coming from the bottom of tank 5 is ready distributed in five tanks (3 to 1) succeeding the ited. No calcium sulphate is present in the ph'osphoric acid produced. No dense coating of cal- Vcium sulphate is produced on the superphosphate granules whereby proper. penetration of acid into the porous'particles is apt to be prevented. Furthermore there are no local accumulations of calcium sulphate and ltherefore there is no ologging of passages between the particles of superphosphate. 'Ih'e leaching that takes place with the above patented superphosphate is believed to be accomplished to a great extent by an osmotic effect. .Strong phosporic acid is formed inside the pores of each granule. Weaker solution coming into contact with the granules diffuses into the pores and is washed out by succeeding streams of leaching liquid. By the end of a cycle the tank through' which water is passed contains spent material containing asA little as 1% P205, the reaction having taken place practically throughout each granule as well as throughout each bed.

Tank 8, in the stage illustrated Jn Fig. '7, has

been lled with fresh superphosphate mixed with jph'osphoric acid solution. The phosphoric acid solution for this purposeis solution that has been collected in tank 82 or 83 during a previous stage. One or the other of the collecting tanks l2 or 83 is always kept full so that when a leaching tank is empty, the solution is ready to' be passed into the leaching tank to ll it halfway. Thereupon to be pumped to production through the pipe 38 that passes to storage. The flow to storage is continued until a predetermined amount of phosphoric acid is removed from the last charged tank in a series. This removed acid is pumped to fill one of th'e tanks 82 or 83 for use in the next charge and the make passed to storage. When a signal sounds for 'the tank being lled, the operator is warned in plenty time to properly direct the flow.

A variation in the use of the tanks 32 and 83 may be referred to, namely, solution from tank 3 (Fig. '7) may be pumped into tank 82 until half full, the flow then switched to tank B3 to conduct a portion to storage, and the flow switched back to tank 82 to fill the latter. i

It is noted that the concentration of the phosphoric acid solution produced by the present process is the equivalent of over 40% P205 and this may be as high as 50%. In fact the amount of phosphoric acid passed to production in any given type of cycle regulates the concentration of the product.

In the stage of operation illustrated in Fig. 8, phosphoric acid solution is being pumped to production from the bottom of the newly charged tank i. Tank 2 is being emptied and solution from the bottom of tank 8 is being pumped into the top of tank `l while water is passed into tanky 3 and acid in the intermediate tank 4 8. Fig, 9 illustrates the next stage with the above steps advanced by one tank. v In Ashutting down the operation of the plant the starting operations previously described may be reversed, whereby the leaching in each tank is eventually completed.

No special distributors for applying liquid over the beds are necessary. Th'e liquid applied readily proceeds down through the relatively deep beds without channelling. The particular superphosphate granules referred to retain their. strength and shape throughout theextraction.

process so that free passage of liquid through a bed is maintained. Besides rendering filtration of Athe phosphoric acid product unnecessary, evaporation thereof is likewise not necessary. It is ready for commercial use as for instance in the manufacture of triple superphosphate.

The invention as hereinabove set forth is embodied and practiced in particular manners but may be variously embodied and practicedwithin the scope of the following claims.

What is claimed is:

1. A process of manufacturing phosphoric acid, comprising submerging in an aqueous sulfuric acid leaching medium a quiescent bed containing preformed superphosphate granules for effecting diffusion of the said medium throughout the depth of the bed and to form phosphoric acid and calcium sulphate, and withdrawing from the bottom of said bed-said phosphoric acid dissolved in said aqueous medium.

2. A. process of manufacturing phosphoric acid comprising submerging in an aqueous sulfuric acid leaching medium a relatively deep quiescent bed containing preformed superphosphate granules to form phosphoric acid and calcium sulphate, and withdrawing from said bed said phosphoric acid dissolved in said aqueous medium.

3. A process of manufacturing phosphoric acid comprising treating a relatively deep quiescent bed of preformed granules of superphosphate with sulphuric acid to form phosphoric acid `and calcium sulphate in the granules, passing water through said bed oi acid-treated granules and withdr `a water containing dissolved phosphoric acid from said bed.

il. A process of manufacturing phosphoric acid comprising submerging in sulphuric acid dissolved in an aqueous medium a relatively deep quiescent bed containing preformed superphosphate granules to form phosphoric acid and calcium sulphate, and while maintaining said bed so submerged and said granules stationary with respect to each other passing said aqeuous mediurn into said bed and withdrawing phosphoric acid solution from said bed.

5. A process oi manufacturing phosphoric acid comprising forming a deep bed of preformed supcrphosphate granules in a leaching tank, maintaining said bed in a quiescent state covered with sulfuric acid so as to react with said granules and form phosphoric acid and CaSOi in said granules, passing water into the tank while the 'hed is submerged and withdrawing phosphoric acid from said bed. f

d. In the manufacture oi' phosphoric acid from superphosphate, subjecting preformed granules of superphosphate in a quiescent bed to the action oi sulphuric acid to form phosphoric acid, pass@ ing an aqueous medium through the' resultint;` acid-treated iced and submerging it in said medium, and removing extracted phosphoric acid. fr the bed while maintaining grannies unremoved from said bed submerged in said medium. 7. In. the manufacture oi' phosphoric acid, the steps comprising depositing preformed superphosphate granules into a body ci sulphuric acid soiution for reacting with the superphosphate to torni phosphoric acid while completely submerged quiescent in said acid solution, continuing the deposition of superphosphate granules until a deep bed is formed in the acid solution. and subsequently passina` water through the hed to remove phosphoric acid. therefrom ii. En the production of phosphoric acid soiution by leaching acid-treated super-phosphate while intaining particles of said superphos phate in a deep bed stationary with respect to each other, the said particles beiner preformed, discrete, indurated nodules oi relatively high compression strength by reason of a combined tumbling and heat-treatment operation; charging an extraction zone with said particles of superphosphate and forming a deep Ibed of said particles in said none, reacting said superphosphate with sulphuric acid in said `bed and completely submerging it in resulting acid solution in said zone, passing water continuously at a predetermined rate into said zone while extracting phosphoric acid from said particles, the said particles retaining substantially their original shape throughout the extraction process thereby maintaining. free passage of liquid throughout the bed. withdrawing extracted phosphoric acid solution irom said zone, and removing the spent particles from the extraction zone.

`9. In the manufacture of phosphoric acid from superphosphate, passing an aqueous extraction medium successively through a plurality of extraction zones containing material resulting from the treatment of preformed granules of superphosphate in unagitated beds in said zones with sulphuric acid'to convert the superphosphate in the granules into phosphoric acid, and while maintaining granules in said acid-treated beds unremoved therefrom quiescent and submerged in said aqueous extraction medium continuing the passage of said medium successively through said zones and withdrawing from an end zone phosphoric acid solution of relatively high concentration.

10. In the manufacture of phosphoric acid from superphosphate, passing an aqueous extraction medium successively, downwardly through a plurality of extraction zones containing preformed granules of superphcsphate in quiescent, bulksome beds that have been treated with sulphuric acid to convert the superphosphate in the granules into phosphoric acid, and -while maintaining said beds submerged and quiescent in said aqueous extraction medium continuing the passage thereof successively, downwardly through l said zones and withdrawing directly from an end zone a phosphoric acid solution of high concentration equivalent to a PiOt` content of over substantially fi0%.

11. In the manufacture of phosphoric acid from superphosphate, contacting unreacted superphosphate an'd sulphuric acid in an aqueous medium in one or a plurality oi interconnected zones containing unreacted superphosphate to convert superphosphate in said one zone into phosphoric acid, passing the resulting solution containing uncharged sulphuric acid successiveiy through. others of said zones and subnierging in i said solution the unreacted superphosphate re-s maining in said latter'zones, the said superphos phate being initially preiorrned grannies thereof phate in a hed stationary with respect to each Y other, the said particles coneprisine,V preformed, discrete, indurated porous nodules of relatively high compression strength by reason of a comu bined tumbling and heat-treatment operation; introducing into said bed, and coverhag with, sulphuric acid dissolved in an aqueous medium to react with said superphosphate and form phosu phoric acid and calcium sulphate in the said nouules, and 'while maintaining said bed so covered, continuously passing 'water into said Ibed to din-'use into the pores of said so-treated nodules by an osmotic eiect between weaker acid outside the nodules and stronger acid within the nodules and continuously removing phosphoric acid solution from said nodules in a stream of inltrated water.

13. In the manufacture of phosphoric acid from superphosphate, passing water through the first of a series of interconnected extraction zones in contact with material resulting from the treatment, in said iirst zone, of preformed superphosphate granules in a deep bed therein with sul- 'phuric acid to convert superphosphate in said granules into phosphoric acid. and while passing assenza sulphuric acid into an intermediate zone in said series in contact with unreacted phosphate material remaining'from such acid treatment of preformed superphosphate granules in a deep bed in the latter zone, passing acid solution from each zone, including the first, into the next succeeding zone in said series containing unreacted phosphate material remaining from such acid treatment of preformed granules of superphosphate in a deep bed in said succeeding zone; and passing acid solution from the last of said series of zones through a zone in contact with unreacted, preformed superphosphate granules, to obtain aqueous phosphoric acid solution.

14. In the manufacture of phosphoric acid from superphosphate, passing water through the nrst of a series of interconnected extraction zones in contact with material resulting from the treatment, in said nrst zone, of preformed super-phos- Dhate granules in a deep bed therein with sulphuric acid to convert superphosphate in the granules into phosphorioacid, and while passing sulphuric acid into an intermediate zonein said series in contact with unreacted phosphate material remaining from such acid treatment of preformed superphosphategranules in a deep bed therein passing acid solution from each zone, in-

' cluding the first, into the nex't succeeding zone in phosphoric acid solution of high concentration.

l5. In the manufacture of phosphoric .acid from superphosphate, passing water at a predetermined rate of flow through the iirst of a series of interconnected extraction zones in contact with material' resulting from the treatment, in said :drst zone, of preformed superphosphate granules in a deep` bed for a predetermined period of time with sulphuric acid to form phosphoric acid in the granules, controlling the rate of flow of said water while passing sulphuric acid at a prede--4 termined rate of flow into intermediate zones in contact with unreacted phosphate material re maining from such acid treatment of preformed superphosphate granules in a deep hed in each zone and while passing solution from each zone including the first at a predetermined rate into a next succeeding zone in said series containing unreacted phosphate material remaining 'from said of interconnected extraction zones in contact with material resulting from the tretament of preformed superphosphate granules in a deep bed for a predetermined period of time with sulphuric acid to form phosphoric acid in the granules, controlling the rate of ow of said water while passing sulphuric acid at a predetermined rate of flow into intermediate zones in contact with unreacted phosphate material remaining from such acid treatment of preformed superphosphate granules in a deep 'bed in each zone and while passing solution from each zone including the first at a predetermined rate into a next suc, ceeding zone in said series, in admixture with the sulphuric acid introduced into such latter zone, passing solutioni'rom the last of said series of zones through a zone at a predetermined rate in contact with fresh superphosphate, the said fresh superphosphate 'being in the form of preformed granules and in a deep bed in said last mentioned zone, and withdrawing directly from the last zone an aqueous phosphoric acid solution of high concentration equivalent to a P205 content of over substantially 40%. 1

1'7.` In the manufacture of phosphoric acid by treating preformed granules of superphosphate with sulphuric acid in a series of interconnected extraction zones in which the material under treatment is maintained in relatively deep beds, the steps comprising contacting a bed in a zone in which fresh superphosphate granules are placed with phosphoric acid solution containing a minimum concentration of suphuric acid obtained from a previous extraction of a hed in said series, withdrawing phosphoric acid oi relatively high concentration from the first-mentioned bed, contacting an intermediate lned in said series with fresh sulphuric acid and admixed phosphoric acid solution from a previously treated bed, washing a bed preceding said intermediate bed with fresh water, and removing exhausted material from the said water-washed Ihed.

18. In' the manufacture of phosphoric acid by treating preformed granules o' superphosphate with sulphuric acid in a series of interconnected extraction zones in each of which the material is maintained in a separate quiescent Aloedffor successive treatment in a cycle, the steps of contacting a bed in said series in said cycle, in which bed fresh preformed superphosphate granules are l placed, with phosphoric acid solution containing acid treatment of preformed granules of superp phosphate in a deep bed, each of said -beds being a minimum concentration of sulphuric acid obtained from a previous extraction in a preceding bed'in said series, withdrawing phosphoric acid of reltaively high concentration from the ilrst-mentioned lbed in the series and as a product from the said cycle, contacting an intermediate toed in said series with fresh sulphuric acid and admixed phosphoric acid solution from a previously tread bed in said cycle, washing with water a bed in a zone in said cycle preceding said intermediate. hed, and removing exhausted material from said zone and from said cycle. refilling said last zone with superphosphate granules, and repeating the cycle, the bed in the said last zone v taking the place of said inst-mentioned bed. 

